Interpretive Summary: The characterization of snow crystal form is of special importance in meteorology, hydrology, avalanche warnings, and interpreting remote sensing data. Photography of the snow crystal form has been conducted since around 1900 using light microscopy. Because of limited resolution and depth-of-field, light microscopy can't be used to characterize such small snow crystals. Using a field procedure developed over several years and low temperature scanning electron microscopy, this technique is able to display a category of crystals called irregular snow crystals. Although light microscopy is not able to provide a clear picture of these crystals, low temperature scanning electron microscopy shows that most of the irregular crystals are aggregates of more than 100 asymmetrical hexagons and they can occasionally occur attached to larger crystals. Metamorphosed forms also occur in the atmosphere. The low temperature scanning electron microscope provides the only means to display these very small crystals.

Technical Abstract:
For nearly 50 years, investigators using light microscopy have vaguely alluded to a unique type of snow crystal known as an irregular snow crystal. However, the limited resolution and depth-of-field of the light microscope has prevented investigators from characterizing these crystals. In this study, a field-emission scanning electron microscope equipped with a cold stage was used to document the structural features, physical associations and atmospheric metamorphosis of irregular snow crystals. The crystals appear as irregular hexagons measuring 69 to 90 um across when viewed from the a-axis. Their length (c-axis) rarely exceeds the diameter. The irregular crystals are occasionally found as secondary particles on other larger forms of snow crystals; however, they most frequently occur in aggregates consisting of more than 100 irregular crystals. In the aggregates, the irregular crystals have their axes oriented parallel to one another and collectively tend to form columnar structures. Occasionally, these columnar structures exhibit rounded faces along one side, suggesting atmospheric metamorphoses during formation and descent. In extreme cases of metamorphoses, the aggregates would be difficult to distinguish from graupel. Frost, consisting of irregular crystals, has also been encountered suggesting atmospheric conditions that favor their growth can also occur terrestrially.